Design & DIY

How to replace a glass pane

Tools used for glazing Photos courtesy of Raj Chaudhry

Replacing a pane in a wooden window is a job that’s easily within reach of most do it yourselfers. It boils down to just three steps: Remove old putty and glass; install new glass in a bed of glazing compound; and glaze the outside to button it up.

Just how the glazing system comes together is shown in the cutaway illustration below. The glass pane floats in an L shaped recess in the sash. It is held in place by a small number of glazier’s points around the perimeter. Flexible glazing putty surrounds and seals the pane to the wood. The outside-facing putty is sloped to shed water. The putty continues around sides and bottom to keep the system water tight and protect the glass from seasonal wood movement.

Illustration of window glazing cutaway
Glazing forms a flexible, weather-tight seal between the glass and the sash. The graphic shows a job that requires one more step to be complete: a layer of paint from the sash face to the window pane.

Out with the Old

To start your repair, remove the old glazing compound, glazier’s points and glass. The tools you’ll need are very basic, a putty knife key among them. Other tools that might prove useful are shown in Photo A. These include a variety of scrapers, a pair of long-nose pliers for pulling glazing points, and a heat gun, for softening stubborn putty.

Tools used for glazing
Useful glazing tools include a painter’s scraper, single edge razor blades and a razor blade scraper, a sharp chisel, a flexible putty knife, a glazier’s 2-in-1 tool, a flat bladed screwdriver, and a pair of long-nose pliers. A heat gun, here with a deflector on the nozzle, works to soften old, hard putty.
Putty knife between wood and old glaze
Sometimes you’ll have luck working a thin putty knife into the joint between the wood members and the old glazing compound. Try to avoid gouging the wood.

Remove any loose shards of glass, and scrape out the remaining putty. With hard, intact compound, look for weak spots where the wood and putty meet. These often occur near the corners and can be telegraphed by cracks in the paint. Try working a thin, flexible putty knife into this seam (see photo above). The putty knife is less likely than a sharper tool to gouge the wood. Follow with a razor blade scraper to separate the putty from the glass (see photo below). For tough jobs, you may want to soften the old putty with a heat gun. Then scrape.

Razor scraping putty from glass
With the bond broken between the wood and putty, a razor blade scraper quickly separates the putty from the glass.

Pull the glazier’s points. Modern eared glazing points (see photo below) are fairly easy to grip with a pair of long nose pliers. Sometimes you’ll encounter flat metal diamonds. They can be harder to get a purchase on. Clean around them. Wiggling them side to side will loosen them. They tend to be soft, so you can usually slip a thin putty knife under the end and bend it up enough to grab with pliers.

Glaziers points
Modern glazier’s points are easy to install and remove. The tabs or ears form a bearing surface for a scraper or similar tool during installation.

Remove any remaining glass and the bedding compound underneath it. Scrape the window recess as clean as you can and remove the dust. Apply an oil based primer to seal it, and allow it to dry.

In with the New

Begin the process of replacing the glass by measuring the opening. Subtract 1/8” from length and width. This will leave a 1/16” gap around the perimeter of the pane for seasonal movement of the sash.

You can take these dimensions to your local hardware store or home centre and have a new pane cut. Or you can cut the glass yourself. Although cutting glass may seem daunting, it is not that hard. It entails scoring the glass and snapping it along this line. If you have a piece of scrap glass, a practice cut or two will build your confidence.

The basic necessities for cutting glass are pictured below. They include a straightedge rule and a marker or pencil that writes on glass, a glass cutter, and glass cutting oil (mineral oil, kerosene or most light oils will do in a pinch).

Tools used for glass cutting
The basic necessities for cutting glass include a straight edge, tape measure, a glass marker (a Sharpie works fine), a glass cutter and cutting oil. You can lay out for genuine glass cutting oil, or substitute mineral oil, kerosene or just about any light oil you have on hand.
Cutter wheel offset
For this particular glass cutter, the straight edge needs to be offset from the score line by about 1/8”

Work on a flat surface. A couple of layers of newspaper can help to even out slightly rough surfaces and provide some cushioning. Draw your cut lines on the glass with a marker and straight edge. To score the glass, you will be sliding the side of the tool along the straight edge. Because the cutting wheel is centred in the tool, you will have to offset your straight edge from the line by this same wheel- to-edge distance (see photo above). The easy way to do this is to set your cutting wheel on the score line and slide the straight edge up against the side of the tool (see photo below). Do this at each end of the line.

Using glass cutting wheel
With the cutting wheel on your score line, you can set the proper offset for your straight edge without any extra measuring. Do this at both ends of the line.

Before you start cutting, lubricate the cutting wheel. Some folks apply a drop or two of oil to the wheel before each cut; some keep a small cup of oil handy and dip the tip from time to time; and some cutting tools have built-in reservoirs that meter oil to the tip.

When cutting, the key is to make a single continuous score from edge to edge. Use even, moderate pressure. Too much causes flaking or splintering, and can even break the glass. Wear gloves and safety goggles.

A classic style of glass cutter has a ball on one end. This feature is used to tap along the back of the score line before snapping.

Tap or not, once you’ve scored your line, snap the glass over the edge of your work surface. Line the score up with the table edge or just barely beyond it. Using two hands, lift the sheet a bit and bring it down evenly. It should break cleanly. Repeat for each dimension you need to cut.

Wipe the new pane with glass cleaner or denatured alcohol to remove cutting oil and any other contaminants that might keep putty from adhering properly.

The Home Stretch

Installing the new glass starts with applying a thin bed of glazing compound, known as back bedding, to the backstop. When working with putty, remove the contents of the can; form the compound into a ball; and knead it thoroughly. This is a critical step. Kneading incorporates any separated liquids, warms the compound and makes it pliable. Depending on your choice of glazing compound, you’ll want either mineral spirits or water and paper towels on hand for cleaning up. The instructions will list the proper solvent.

The method of application is sometimes dictated by the product. A tube-dispensed glazing compound is applied as a continuous bead with a caulk gun. For the more common glazing putties, the choice often comes down to personal preference. Some people like to roll the putty into thin ropes and press these into the recess. Others hold a small ball of glazing compound in one hand and a putty knife in the other and pack on a bit at a time. You can even use your fingers. In any case, ensure that you leave no voids.

Press the clean glass firmly and evenly into the glazing compound, leaving a 1/8” thick bedding layer between wood and glass.

Secure the pane in place with glazier’s points. For most small replacement panes, you can use two points per side, about one-third from top and bottom, plus one point each in the middle of the top and bottom edges. For larger panes, place one every 10” or so. Set them on top of the glass, and press them point first into the surrounding wood using a scraper or similar tool. Rocking or wiggling the blade a bit makes the job easier. A glazier’s two-in-one tool is ideal for this job, because its scraper blade is offset, which helps keep the force out toward the frame, where you want it, and not down onto the glass, where you don’t. (See photo below)

Using a glazier's two-in-one tool
The glazier’s 2-in-1 tool excels at pushing in glazing points. Its cranked blade is stiff and helps direct force out toward the frame and not down, which could compress your soft new bedding.
First rough application of glazing
The first rough application of glazing compound is in place, and well packed, and the final bevel is ready to cut.

Tool down and trim off any compound that has squeezed out on the inside face of the sash so that it is flush with the backstop. You will need to repeat this again at the end of the glazing process.

Next comes the angled glazing on the outside face. Press glazing putty where sash meets glass, ensuring you have a good seal on each surface. You’ll need enough to fill from the backstop line to the top of the sash. After roughly packing the recess (see photo above), and letting it sit for a few minutes to bond, begin cutting the compound to your desired profile. One method is to start at a corner, set the angle of your blade — with one side pressed against the wood and the edge or blade corner set at the rabbet width — and cut straight across to the other end (see photo below). Repeat for each side. If you are working on a loose sash, you can rotate it as you work. Many people like to work with the sash standing up and leaned back, focusing just on the lower horizontal bead in each orientation of the sash. As you progress, make sure you have no gaps or flaws that might let water in. Clean up the corners, where putty can pile up, and smooth as needed. For tube-dispensed glazing, use the 45-degree tip of the cartridge to put an angled bead from the top of the sash to the glass. Then form clean corners and smooth the surface with a putty knife.

Flip the sash over and again cut away any bedding putty that has squeezed out.

Cutting the bevel
Cutting the bevel from one side to the other, with a bit of downward pressure, leaves just some clean-up in the corners and a bit of smoothing.

After removing any cut off putty, you can clean residue from surrounding surfaces using paper towels dampened with mineral spirits or water, depending on your glazing product. But avoid getting solvent too close to the new compound. It’s best to let this set for several days and then remove any excess material and haze with a single edge razor blade or similar tool.

The final step is paint. It can take from several days to a month for new glazing compound to skin over and achieve a firm set, which are prerequisites for painting. Check your instructions for specific guidance. Paint is an important part of the weather-sealing system. The paint should extend across the joint between sash and glazing compound and about 1/16” onto the glass.

12 tips for painless panes

It is not always necessary to remove a window sash to replace a broken pane. But it usually is advisable. It’s much easier to get great results on a bench top than on a ladder – safer too. You can rotate the loose sash to find the best working angle; you can get better leverage; and you won’t have to worry about your fresh compound slumping or drying too quickly.

Most glazing compound manufacturers offer downloadable technical bulletins for their products. These typically offer more detailed instructions than those found on the side of the can, and they are much easier to read.

Safety goggles and gloves
Wear gloves and eye protection to guard against injury from sharp glass.

Wear safety goggles and gloves when working with glass.

Plan your work so that you have a several-day window of dry weather.

Old paint may contain lead. Some very old formulations of glazing putty also used lead as an ingredient. Work in a well ventilated area, and take appropriate precautions. If you are concerned, wear a HEPA mask and use a vacuum equipped with a HEPA filter for dust. After vacuuming, and before setting your new window pane, wash the sash with soap and water, and allow it to dry thoroughly.

If repairing a window in place, you can keep glass shards and other debris from falling into the cottage by taping craft paper or newspaper over the inside of the opening. To help prevent broken glass from falling out, particularly important with large panes, place an X of duct tape over the glass.

Keep your putty knife clean and polished. You’ll never get a smooth finish from a rough, rusted or dirty blade. For latex compounds, wipe the knife blade regularly with a damp rag. For oil- based putties, use mineral spirits in place of water. This will also keep putty from sticking or grabbing.

If you use heat to soften old putty, do so judiciously, and keep your heat gun moving. Never apply heat to a cold window. Various nozzle attachments are available that can help direct hot air toward the putty and not the glass.

Paint is part of the weather sealing system. So don’t skip this step. The paint should extend over the glazing compound about 1/16” onto the glass.

It often takes a couple of months for glazing compounds to cure completely. During this period, be gentle with the window. Don’t bang it open or closed. Newly applied glazing compounds need to skin over before they can be painted. This can take days or weeks, depending on the product used and the type of paint applied. Consult the manufacturer’s instructions.

Opinions vary on the best types of glazing compounds. For most DIY jobs, the common products you’ll find in hardware stores work just fine. For longevity, many pros still swear by old school putties made with linseed and vegetable oils. They can be pricey but certainly are worth considering, especially for restorations or for glazing a window in place. They have excellent workability and resist sagging. Some skin over quickly, which means they can be painted sooner. And some dry very slowly, which is a virtue when applied outdoors. You can keep leftover linseed oil putty from drying out by freezing it. Roll it into balls and store in freezer bags with the air squeezed out.

Match your new window glass, when possible, to the surrounding panes. Thickness and coatings can vary. Salvaged sashes with vintage glass are often available very reasonably on local classified sites. For a really old house, with wavy glass, you may want to visit an architectural salvage shop for an antique sash to cannibalize. You’ll have matching replacement glass the next time you need it. Online specialists also sell reproduction glass.

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